Fluid circulation in the arm enters via arteries but leaves via two
different pathways.  The blood from the arterial side goes through
capillaries, where a portion of the water and dissolved material leaks out
into the space between cells, becoming lymph.  Most of the blood goes back
to the heart via veins, but the fluid that leaked out of the capillaries
goes back into thin-walled vessels called lymphatic channels or vessels.
The fluid filters through lymph nodes.  Damage to the nodes, from surgery or
from chemo or radiation, impairs the return through this system, and the
fluid accumulates in the arm.

It may be that the symptoms will subside very slowly as healing from this
lates round progresses.  It might not, or might improve without going away.
In the meantime, an elastic sleeve can be purchased or, better yet, made to
her exact size.  In the US, the most common brand is Jobst, but there are
other good manufacturers.  Someone needs to measure her arm and hand (an
elastic glove is usually separate from the sleeve).  Jobst does that by
means of a strip of paper with perpindicular bands spaced allong it like
teeth of a comb.  You run the strip up the arm then tape the side bands
around the arm with cellophane tape, cut through the bands with a pair of
scissors, then fold the mess up and send it off.  The sleeves are hard to
get on; you will need cornstarch or talcum powder and it might even be a
two-person job.

There are other precautions.  Blood should not be drawn from that arm nor
intravenous fluids administered on that side.  Blood pressure should be
checked on the other side.  Infections in that arm should be treated quickly
and aggressively; the lymph is an excellent growth medium for bacteria and
infection can get very serious very quickly.  When prractical, sitting with
the arm elevated will slow the accumulation of fluid and speed is drainage.